1. Field of the Invention
The present invention relates to a positron source, and more particularly, to a positron source suitable for use in an electron microscope.
2. Discussion of Relevant Art
A scanning electron microscope is described in European Patent EP-B1 0 205 092, with which investigation of a sample can take place selectively with electrons or with positrons, and the sample can be selectively irradiated with electrons or positrons for this purpose. In an embodiment that is described, the positron beam and the electron beam are superimposed on each other by a deflecting magnet and subsequently conducted along the optical axis of the magnetic lens of the scanning microscope. The radioactive nuclide .sup.22 Na, which is largely e+ active, is used as the positron source. An aluminum foil is positioned in front of the radioactive nuclide in the beam direction; the positrons emitted from the nuclide are braked by it to thermal energy with an energy width of one electron volt. This source has a relatively large spatial extent, and the phase space of the positrons is therefore substantially greater than the phase space of the electrons. Furthermore, the yield of this kind of moderation of the positrons is very low, since only positrons that diffuse through the whole of the aluminum foil emerge again on the other side of the aluminum foil. The intensity of the positron source is very weak because of this low yield, and long measurement times are thus required for measurements of the electron/positron recombination in the sample. In addition, a further weakening of the positron beam takes place within the imaging optics of the electron microscope, in that a large portion of the positrons is absorbed by the tube of the electron microscope in the region of the imaging optics.